1. Field of the Invention
The present invention relates generally to differential pulse encoding and decoding approach for binary data transmissions. More particularly, the subject invention pertains to methods and circuits for differential pulse encoding and decoding of data for binary data transmissions, such as binary frequency shift keying (BFSK) data transmissions, for sending and recovering a serial, binary digital data stream by differentiating the pulses thereof.
2. Discussion of the Prior Art
Problems exist in the prior art with respect to modulating a binary digital data stream directly onto the carrier frequency of a frequency modulation transmitter, which is generally referred to as Binary Frequency Shift Keying (BFSK). Several derivatives of BFSK are also known such as Minimum Shift Keying (MSK) and Gaussian. Minimum Shift Keying (GMSK). These derivatives have a narrower spectrum than that of the basic BFSK. However, none of these approaches has solved an intrinsic problem of BFSK, which is that the data stream must not stay at a high level for a long period of time. In other words, the data stream must be randomized before being forwarded to the frequency modulator. Sometimes the data stream cannot be randomized as desired, particularly if it has many contiguous bits at one logic level, which results in higher data transmission errors. The cause of this problem is that most modern FM transmitters use a Phase Locked Loop (PLL) to maintain the carrier frequency at a preset frequency. Any deviations of the carrier frequency therefrom cause the PLL to function to adjust the frequency back to the preset frequency, which means that the PLL essentially works against the BFSK data transmission arrangement. Because a time gap exists in the PLL between the time when the PLL detects a frequency change and the time it starts to correct the frequency, BFSK can still be used effectively provided that the bit logic level changes faster than the time gap. The method and circuit of the present invention totally solves this intrinsic problem of BFSK data transmissions.
The method and circuit of the present invention also solves a problem with electromagnetic interference encountered during transmitting a binary data stream using a cable medium. In this case, the normal rectangular data pulses create higher order harmonics which in turn produce high levels of electromagnetic noise. The present invention converts the rectangular data pulses into a stream of sine wave like pulses which consists of very few high-order harmonics.